* include/wget/wget.h: Remove wget_hashmap_get_null() and
  wget_stringmap_get_null(),
  change params and return value of wget_hashmap_get() and
  wget_stringmap_get().
* libwget/hashmap.c: Remove wget_hashmap_get_null(),
  change params and return value of wget_hashmap_get(),
  simplify hashmap_find_entry().
* libwget/stringmap.c: Remove wget_stringmap_get(),
  change params and return value of wget_stringmap_get().
* libwget/dns_cache.c: Amend for new API
* libwget/hpkp.c: Likewise
* libwget/hsts.c: Likewise
* libwget/http.c: Likewise
* libwget/netrc.c: Likewise
* libwget/ocsp.c: Likewise
* libwget/tls_session.c: Likewise
* src/host.c: Likewise
* src/options.c: Likewise
* src/plugin.c: Likewise
* src/stats_site.c: Likewise
* unit-tests/test.c: Likewise